In the conventional mobile cellular communication systems, when OFDM (Orthogonal Frequency Division Multiplexing) using spreading codes, that is, a 1-frequency iteration cellular system based on OFCDM is realized, a technique of reducing the amount of interference by converting radio waves from cells to noise using scrambling codes and separating a signal through despreading on the receiving side, is known (for example, Patent Document 1). Further, in a mobile cellular system where a cell of a base station apparatus has a finite radius, a mobile station needs to sequentially switch base stations as the mobile station moves, so that the mobile station requires a means for knowing base station apparatuses of switching candidates. Therefore, the mobile station uses, for example, a method of having a base station apparatus within its communication area report information of another base station apparatus around the mobile station as report information, and thereby knows peripheral base station apparatuses of the switching candidates. The mobile station then actually receives radio waves of the base station apparatuses of the switching candidates and measures a radio wave channel condition, and thereby determines whether or not it is possible to perform handover to the base stations of the switching candidates. In this way, to smoothly realize handover around a cell boundary, the mobile station measures in advance the reception condition of a base station apparatus predicted to be a handover destination (hereinafter, described as an “active set”).
A cellular system uses a code spreading scheme (CDMA) whereby in each cell, the transmitting side performs spreading processing and the receiving side performs despreading processing using spreading codes. When a code spreading scheme is used, even when an identical frequency is used between cells, signals between cells can keep orthogonality, so that identical frequency can be repeatedly used between cells. Such a system is called a “1-frequency iteration cellular system.”
Furthermore, according to a multicarrier transmission scheme such as an OFDM modulation scheme, the transmitting side modulates an information signal using a plurality of subcarriers and inserts a guard interval in the transmission signal for the purpose of reducing waveform distortion due to multipath delay waves.
Further, in mobile communication, frequency differences are produced due to a Doppler frequency according to movement speeds between base station apparatuses and a mobile station. In addition, the Doppler frequency varies between one mobile station and each base station apparatus. When a relative speed between the base station apparatuses and the mobile station is V, the speed of light is C and a communication carrier frequency is f0, Doppler frequency fd can be calculated as fd=(V×f0)/C. When, for example, the relative speed is 300 km/h in a 4-GHz carrier wave, the frequency difference due to the Doppler frequency becomes a value exceeding 1 kHz. As for a relative speed between two base station apparatuses and a mobile station, when the relative speed between one base station apparatus and the mobile station is +300 km/h and the relative speed between the other base station apparatus and the mobile station is −300 km/h, a frequency difference of 2 kHz or more is produced in received radio waves at the mobile station from the two base station apparatuses.
Further, when an OFDM reception apparatus performs a discrete Fourier transform, if the frequency varies, the orthogonality is lost, and the demodulation accuracy degrades. Consequently, by acquiring a desired cell frequency at an RF section of the OFDM reception apparatus, the OFDM reception apparatus needs to correct the frequency difference in the received radio waves including a Doppler frequency or the like produced between the base station apparatuses and the mobile station. Furthermore, a circuit for detecting reception timing of a scrambling code at the mobile station sets FFT (fast Fourier transform) timing per pilot signal of each active set and detects a reception timing of each pilot signal. In this case, when one circuit detects reception timings of pilot signals of all active sets and the pilot signals of the active sets which overlap with each other in time are received, if a frequency difference of one active set is corrected, a frequency difference of another active set having a different frequency difference cannot be corrected with the corrected frequency. As a result, reception timings of pilot signals of another active set are not detected correctly. This is because the pilot signals of another active set are subjected to FFT with a shifted frequency and the orthogonality is lost. Therefore, when an OFCDM 1-frequency iteration cellular system is realized, measurement of an active set requires a frequency acquisition circuit on a per active set basis and an FFT circuit for detecting a synchronization timing. Here, the “scrambling code” is a long-period spreading code and is a code which varies for each cell, and scrambling codes are orthogonal to each other among the cells.
In an actual third-generation mobile communication cellular system, a mobile station measures SIRs of received signals of eight active sets at a maximum. In a cellular system using OFCDM, when the mobile station is required to receive synchronization signals from a plurality of active sets as in the case of the third-generation mobile communication cellular system, the mobile station needs to be provided with a plurality of sets of frequency acquisition circuits to demodulation circuits in parallel.
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-152681